Journal of Catalysis 184, 1–4 (1999) Article ID jcat.1999.2485, available online at http://www.idealibrary.com on PRIORITY COMMUNICATION Outstanding Performances of Magnesia-Supported Platinum–Tin Catalysts for Citral Selective Hydrogenation Sandro Recchia, ∗,1 Carlo Dossi,† Nicola Poli, ∗ Achille Fusi, ∗ Laura Sordelli, ∗ and Rinaldo Psaro ∗,1 ∗ CNR Centre “CSSCMTBSO” and Dipartimento di Chimica Inorganica, Metallorganica e A nalitica, Via Venezian 21, 20133, Milan, Italy; and †Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Universit` a dell’Insubria, Via L ucini 3, 22100 Como, Italy Received March 4, 1999; revised March 16, 1999; accepted March 16, 1999 Magnesia-supported Sn–Pt catalysts were prepared by reac- tion between preformed platinum metal particles and Sn(n-C 4 H 9 ) 4 . HRTEM and H 2 -chemisorption data confirm the synthesis of very small tin-decorated platinum particles (∼1.0 nm mean diameter). Preliminary studies on catalytic hydrogenation of citral in a trickle- bed reactor reveal that 97%selectivity to unsaturated alcohols at 100%conversion can be easily achieved. The electronic ligand ef- fect produced by the basic support is supposed to be fundamental to enhance the selectivity to unsaturated alcohols and to stabilise the bimetallic particles. c  1999 Academic Press Key Words: Pt–Sn catalysts; Pt–Sn/MgO; α,β -unsaturated alde- hydes (hydrogenation); citral (hydrogenation); geraniol synthesis; nerol synthesis. INTRODUCTION The development of heterogeneous catalysts for the se- lective hydrogenation of α,β -unsaturated aldehydes to the correspondingunsaturated alcoholshasattracted much in- terest for their potential utilisation in the production of valuable intermediates. High selectivities are strongly re- quested since the conventional stoichiometric processes generally lead to 100% production of unsaturated alco- hols. From the literature data clearly emerges that the Os, Ir, and Pt-based systemsshowa remarkable selectivityin these reactions (1). To further improve the selectivity to unsat- urated alcohols of supported-metal catalysts, the introduc- tion of promoters (IA-IIA elements, transition metals, or s,p-block elements) is necessary. In this respect Pt–Sn and Pt–Ge based catalysts are probably the systems where the best promoting effects are achieved (2). It was, however, reported for different Pt-modified systems that fast ageing 1 To whom correspondence should be addressed. Fax: 39-02-70600630. E-mail: crosaro@csmtbo.mi.cnr.it. processes take place while working in flow conditions (3), thus limiting the working lifetime of catalysts. A pulsed- flow strategy was recently proposed to overcome this prob- lem (4), but even in that case the selectivity to unsaturated alcohols was never better than about 65% at 40–50% con- version. The challenge is to obtain a highly selective and active catalyst, which also shows a high stability in terms of cata- lytic performances and which, hopefully, regains the initial properties after an appropriate regeneration process. Our approach to meet these requirementsconsistsofthe utilisa- tion of MgO as the support to obtain electron-rich metallic particles. In the case of palladium it is in fact reported that the utilisation of magnesia leads to the formation of very small electron-rich supported metallic particles (5). In this respect it must be pointed out that different authors (1) re- ported that a shift ofelectron densityfrom the support to the metalparticle should be verylimited in the space,and hence no variation should be expected from the catalytic point of view. However, with very small metallic particles, say about 1nm,electroniceffectscould playa fundamentalrole.Since it was recently shown that such very small Pt particles can be synthesised on supports like SiO 2 and hydrotalcite (6), we have decided to develop a novel organometallic-based Pt–Sn/MgO catalyst. It is important to note that the surface organometallicchemistryon metalscan be a veryclean way to selectivelyadd a second metalat the surface ofa first one. By using this approach Claus et al. (7) achieved the highest selectivity to date for crotyl alcohol (85% selectivity at 65% conversion) in the gas phase hydrogenation of crotonalde- hyde. In this preliminary communication we will present the outstanding catalytic performances obtained with these new Pt–Sn/MgO catalysts in the selective hydrogenation of citral that was chosen as a model reaction and carried out in a trickle-bed reactor. Flow reaction conditions were adopted to better evaluate stability and regenerability of the catalyst. 1 0021-9517/99 $30.00 Copyright c  1999 by Academic Press All rights of reproduction in any form reserved.